1. Field of the Invention
The present invention relates to an apparatus and method for improving the economics of hydrocarbon production from a producing well. In particular, the present invention relates to an apparatus and method for selectively lifting produced fluid, including produced hydrocarbons and a portion of produced water, to the ground surface and for injecting the remaining produced water, subsurface, in a subterranean well.
2. Related Art
Conventional hydrocarbon production wells have been constructed in subterranean strata that yield both hydrocarbons, such as oil and gas, and an undesired amount of water. These wells are usually lined with heavy steel pipe called "casing" which is cemented in place so that fluids cannot escape or flow along the space between the casing and the well bore wall. In some wells, large amounts of water are produced along with the hydrocarbons from the onset of production. Alternatively, in other wells, relatively large amounts of water can be produced later during the life of the well.
The production of excess water to the ground surface results in associated costs in both the energy to lift, or "produce," as well as the subsequent handling of the excess produced water after it has arrived at the surface. Moreover, the produced water must be disposed of after it has been brought to the ground surface. Surface handling of excess water, in addition, creates risks of environmental pollution from such incidents as broken lines, spills, overflow of tanks, and other occurrences. Further, the facilities, lines, and wells required to handle excess water disturb the environment by virtue of their construction and presence. Accordingly, many oil production fields and wells often rapidly become uneconomic or environmentally hazardous to produce because of excessive water production.
Various apparatuses and methods have been proposed to overcome the problems associated with excess water production and the aforementioned problems associated with lifting, or producing, this water to the ground surface. Several approaches have been used to produce excess water to the ground surface or to avoid producing the excess water to the ground surface by shutting off the water at the entry into the wellbore. Among these means are: installing larger pumps to pump the water to the ground surface; shutting off the water by injecting gels or resins into the formation; and installing mechanical means in the well to interrupt the flow of water into the wellbore. These approaches, however, have not recognized that effective removal of water from oil or gas wells can be accomplished by transferring the accumulated water subsurface to a water-absorbing injection formation.
An evolving approach to the problem of excess water production is to take advantage of the downhole gravity segregation of produced hydrocarbons and produced water in the wellbore. The excess produced water is then conveyed into an injection formation of the subterranean strata while, for example, the oil and a small portion of the produced water that has not fully segregated from the oil are produced, or "lifted," to the ground surface. Such an approach has generally been referred to as an "in-situ" injection method. The conveyance downhole of produced water, without having lifted a majority or all of it to the ground surface, can substantially improve lease revenues or reduce lease operation expenses and investments, thereby extending the economic life of entire fields.
Devices or systems that lift and/or flow hydrocarbons and a portion of the water to the ground surface, while simultaneously injecting the water which has been separated downhole, may be referred to by those persons having ordinary skill in the art as "Dual Injection and Lifting Systems (DILALS)," or alternatively, as "Downhole Oil Water Separation (DOWS)."
Generally, such methods have required the availability of a suitable injection formation, either below or above the production zone, with sufficient permeability to permit injection of the excess water into the injection formation. In addition, these in-situ methods have generally employed coupled pumps of the same type (e.g., dual rod pumps). These pump combinations have generally been powered by the same prime mover or drive unit, such as a conventional pump drive located at the ground surface.
Conventional coupled systems which have been driven by the same prime mover have presented numerous problems with regard to production flexibility in order to accommodate changing reservoir conditions. This is so because it has not been feasible or simple enough to individually control the amount of fluids being lifted to the ground surface and the amount of water being injected by the coupled pumps. For example, the output of the lifting pump in a coupled system, such as a dual-rod pump, may not be variably reduced during production and the output of the injection pump may not be variably increased during production. Such flexibility is needed, for instance, when the well volume remains constant during production but the percentage of oil production decreases with time.
One example of a conventional production apparatus of the coupled in-situ type is a Dual Action Pumping System ("DAPS") that produces oil and a portion of the water from a casing/tubing annulus on the upstroke of the pump, injects water on the downstroke, and uses the gravity segregation of the oil and water within the annulus. Such an apparatus is shown in U.S. Pat. No. 5,497,832, also assigned to the assignee of the present application, the entirety of which is incorporated herein by reference.
Tests of this technology in a number of different wells have shown that gravity segregation of oil and water enables a dual-ported, dual-plunger rod pump to selectively lift produced fluids, including produced hydrocarbons and a portion of produced water, while separating and injecting the remaining produced water into an injection zone within the subterranean strata.
The DAPS apparatus, however, does not solve all of the problems associated with excess water production or changing water production within the subterranean reservoir. Very often, the use of two pumps of the same type (e.g., dual rod pumps) may limit the ability of the pumping system to minimize the amount of water lifted to the ground surface. For example, a system, such as DAPS, using a 1.75" diameter rod pump and a 1.5" diameter rod pump will generally lift approximately 18% of the total produced fluids to the ground surface even though the well produces only approximately 5% oil. Further, in coupled systems (i.e., pumps sharing the same prime mover), as noted above, the ability of the systems to adjust to changing water cut production is limited. For example, the various parts of the pump assemblies of coupled systems cannot economically be changed frequently enough to meet changing reservoir conditions.
In a further example of the conventional in-situ approach, coupled rod pumps are used for separating and producing oil from water in a well, while simultaneously injecting the water into the producing formation or into an injection formation below the producing formation. Such an apparatus is shown in U.S. Pat. No. 5,697,448. The apparatus employs three spaced packers (upper, middle, and lower). An oil pump is located between the upper and middle packers, and a water pump is located between the middle and lower packers. Produced oil and water are accumulated between the upper and middle packers. The oil is delivered through an opening into the oil pump and fills a cylinder associated with the oil pump. Produced water is allowed to drain through additional passages into the water pump cylinder where it accumulates for injection. Selective pumping of the oil on the upstroke of the pump and the water on the downstroke of the pump is effected by a set of check valves associated with both the oil and water pumps. Such an apparatus, however, is not an optimal solution to the problems associated with changing water and oil production presented by conventional coupled systems. For example, the apparatus does not provide the flexibility needed to vary the percentage of total reservoir output that is lifted or brought to the ground surface without substantial modifications to the system.
In another example of an in-situ type apparatus, a formation injection tool, mounted to a bottom-hole tubing pump, carries out underground separation and down-bore in-situ transport and injection of the undesired fluids into an injection formation in the production well. Such an apparatus is shown in U.S. Pat. No. 5,425,416. As with the apparatus shown in U.S. Pat. No. 5,697,448, this system does not provide the flexibility needed to quickly and inexpensively change the proportion of fluids lifted to the ground surface as conditions within the subterranean producing strata change.
Thus, there is a need in the art for an apparatus and method that substantially obviates one or more of the limitations and disadvantages of conventional pumping systems. Particularly, there is a need for a system for lifting produced oil and a portion of the produced water to the ground surface, while injecting the remainder of the produced water into an injection formation. There is a particular need for uncoupled systems which have the flexibility to vary the proportions of fluids lifted to the ground surface to the amount of water injected subsurface within the subterranean strata.